1. Field of the Invention
The present invention relates to a method for determining the voltage sensitivity of the distance between the mirrors of a Fabry-Perot interferometer. The invention also relates to a computer-software product for implementing the method.
2. Description of Background Art
Electrically adjustable Fabry-Perot interferometers (FPI) are commonly used as adjustable band-pass filters in, among other things, optical content measurements. The use of this method achieves considerable improvements in performance and especially in long-term stability. The use of the method according to the invention permits the control-voltage sensitivity of the distance between the mirrors of an FPI band-pass filter to be determined precisely, thus further improving the measurement accuracy of FPI technology.
For example, the long-term stability of the measurement of carbon dioxide (CO2) is based on measuring two wavelength bands in a single channel, an absorption band (ABS 4.26 μm) and a reference band (REF 3.9 μm). These two bands are typically selected using an adjustable micro-mechanical optical FPI band-pass filter, the pass band of which can be selected using voltage. The FPI control voltages corresponding to these bands are marked Vabs and Vref. The content, or a quantity proportional to it is calculated on the basis of the ratio (Tx/Rx) of the signals received from these two bands, i.e. the Tx signal value is measured using the Vabs voltage and the Rx correspondingly using the Vref voltage.
For example, the first equivalent condition for the stability of a carbon-dioxide transmitter is for the voltage control corresponding to the FPI voltages stored in the memory during factory tuning, i.e. the absorption and reference voltages and the pass bands corresponding to them, to remain unchanged over the short and long terms. For example, instability in the control voltage and rapid changes in temperature may lead to shift in the FPI pass bands and thus cause changes to appear.